1. Field of the Invention
The present invention relates to a process for cleaning a harmful gas. More particularly, it pertains to a process for cleaning a basic harmful gas such as ammonia, amines or the like which is used in the semiconductor manufacturing industry.
2. Description of the Related Art
With the continuous development of the semiconductor industry, the optoelectronics industry and the precision machinery and instrument industry, there has been a steady rise in recent years in the amount of basic gases such as ammonia, amines or the like which is used in such industries. The above-mentioned basic gases are each indispensable materials, in the production of a semiconductor, protective films, ultra-hard machinery and instruments, ornamentals or the like which production utilizes a chemical vapor deposition. Such gas is highly toxic and when discharged in the atmosphere, it releases an irritating odor, thus exerting adverse influence on humans and the environment.
With regard to the maximum permissible concentration of each of the gases to humans and in the atmosphere, it is reported as being 25 ppm for ammonia and 10 ppm for trimethylamine. Thus it is necessary to remove the harmful component in the gas after being used in the aforesaid semiconductor production process prior to discharge in the atmosphere.
The aforementioned basic gases are usually available on the market in the state that the gas is filled in 0.1 to 50 liters cylinder.
In order to prevent the atmospheric air from being widely polluted with a gas in case of its leakage from a cylinder, the cylinder is used in the state that it is connected to a gas supply piping to a semiconductor process while it is housed in a cylinder accommodation vessel (a so called "cylinder box") that is connected to a ventilation duct. However, even if a cylinder is housed in a cylinder box, there is no secured complete prevention of such dangerous accident that the cylinder is emptied within only about 5 to 10 minutes by a sudden leakage of the gas therefrom. Under such circumstances, there is eagerly desired a complete and thorough countermeasure capable of coping with such accident to make the harmful gas completely harmless.
As a process for cleaning a basic gas that has been discharged in the atmosphere, there are available two kinds of processes, one being a wet process in which the basic components are absorbed in an absorbing solution composed principally of an aqueous solution of an acidic component by the use of a scrubber; and the other being a dry process which make uses of activated carbon, a porous adsorbent of inorganic compound type, etc.
However, the above-mentioned wet process suffers the disadvantages that when the equipment used therefor is large-sized, it finds difficulty in post-treatment, it requires a considerable expense in the maintenance of the equipment and further that the absorption efficiency is not necessarily high, thereby making it impossible to immediately and completely treat a highly concentrated harmful gas at the present time. In addition, there are employed not only an ammonia-based gas but also other harmful gases at the same time, for example, a silicon compound-based gas typified by a silane in the semiconductor industry and thus, the wet process brings about the problem of sludge formation due to the generation of a powdery oxide.
The dry process exemplified by the above-mentioned absorptive treatment with activated carbon is disadvantageous in its low removing capacity of harmful components and further requires a countermeasure against the possible hazard of causing fire when an inflammable gas such as a silane coexits.
Such being the case, there has long been desired the materialization of a process for cleaning a harmful gas, especially that discharged from the semiconductor industry, said process being enhanced in both treatment rate and treatment capacity for harmful gas, excellent in removal performance, not only in an emergency when a large amount of a harmful gas is leaked from a gas cylinder because of an abnormality, but also in the ordinary case where a harmful gas is discharged in high concentration after being used in a semiconductor production process, and free from the possibility of causing the hazard of fire in the case of gas cleaning or clogging in a cleaning column due to sludge formation.